Helium Extraction from Natural Gas

ABSTRACT

A crude helium stream is recovered from a natural gas feed by distillation. Refrigeration from expanding a portion of the bottoms liquid is used to partially condense the helium-enriched overhead vapor and generate a crude helium vapor and a helium-containing liquid stream that is recycled to the distillation column to maximize helium recovery. The helium-depleted natural gas stream can be returned at pressure for utilization or transportation.

BACKGROUND

The present invention relates to processes and apparatuses for the extraction of helium. In particular, the invention relates to the separation of helium from a natural gas stream comprising methane, nitrogen, and helium using cryogenic distillation.

Helium exists in many natural gas deposits worldwide, but there is a growing interest in efficiently recovering helium from natural gas deposits with low concentrations of helium, e.g. below 2000 ppmv. Recovery of helium from natural gas at these low levels has long been considered uneconomical. Helium recovery from natural gas occurs normally as a by-product of liquefied natural gas (LNG) production or nitrogen rejection. In both cases methane is condensed and the lighter helium is easily recovered as a gas. The present invention relates to the case in which the natural gas stream does not require liquefaction or nitrogen rejection. In this case, the gas may still contain significant nitrogen, but not enough to prevent the natural gas from being used in a pipeline or gas turbine.

Helium extraction from natural gas is known. Gottier (U.S. Pat. No. 5,011,521) teaches helium extraction using a stripping column to enrich the helium concentration above the feed gas composition. Helium enrichment is limited to the action of the stripping column, in the example given as roughly one order of magnitude, from 0.44% to 5.16% helium. The aim of enriching helium in the overhead stream is to reduce the flow to the helium purifier by increasing the helium molar fraction. No additional means to enrich the helium in the stream leaving the top of the stripping column prior to entering the purifier are disclosed.

Gottier also discloses the use of a dense fluid expander (DFE) to recover energy from expanding a higher pressure stream to a lower pressure to feed a distillation column. Operating the distillation column at a higher pressure incurs higher capital costs due to the difficulty of effecting a separation at high pressure and the complexity of supplying reboiler duty to the distillation column. The difficult separation results in a higher reboiler duty for a given helium recovery, which causes a higher vapor flow rate. The higher vapor flow rate coupled with unfavorable surface tension and vapor-liquid density ratio leads to larger column diameters. To avoid these disadvantages, the feed pressure is reduced prior to entering the distillation column.

Oeflke (US2014/0137599) teaches an additional separation to further enrich the helium content of the overhead stream from the stripping column. The overhead stream is cooled and reduced in pressure to form a helium-rich vapor stream and a helium-depleted liquid stream. The helium-depleted liquid stream, which still contains some helium, is pumped and combined with the helium-depleted natural gas from the bottom of the stripping column. The helium not recovered from the helium-depleted liquid stream reduces overall recovery by 0.4% according to the example given. Furthermore, the pressure of the helium-rich vapor stream is reduced from 550 psia to 100 psia in the example which may require recompression to enter the downstream helium purification step.

Mitchell et al (U.S. Pat. No. 4,758,258) teach a multistage separation for recovery of helium from natural gas along with separation of ethane, propane, and heavier hydrocarbons from the bulk methane. It is similar to Oeflke in two respects. First, the refrigeration for the final separation of helium and nitrogen from methane is achieved by reducing the pressure of the feed to the separator to produce a crude helium stream. Second, the helium contained in the liquid stream from the separator is not recovered, reducing the overall helium recovery.

Agrawal (U.S. Pat. No. 5,167,125) teaches a process where light gases, such as helium, are removed by partially condensing the overhead vapor from a distillation column. The liquid stream formed provides reflux to the distillation column and the helium-enriched vapor stream can be further purified.

In order to minimize the power required in helium extraction processes described in the prior art, intermediate streams that contain small but significant amounts of helium are rejected to the helium-depleted natural gas product, lowering overall helium recovery. There is a need for achieving the highest possible overall helium recovery by recovering helium from intermediate streams in a power-efficient manner.

SUMMARY

This invention relates to a multi-step process to extract helium from a natural gas stream optimized for high helium recovery and low power consumption. First, contaminants are removed as needed, for example CO₂ by amine absorption, water and heavy hydrocarbons by temperature swing adsorption, and/or mercury by adsorption on activated carbon. Next helium is extracted using a cryogenic distillation column system. The helium content in the column overhead stream is enhanced with a condenser to recover nitrogen and methane, both increasing methane recovery and reducing the flow rate to downstream helium purification. The crude helium stream passes to a cryogenic partial condensation process to further increase the helium concentration before hydrogen is removed by catalytic combustion. Final purification is by pressure swing adsorption (PSA), from which the tail gas is recompressed, dried and recycled. The pure helium product from the PSA can then be liquefied for transport and sale.

The helium-depleted liquid from the bottom of the distillation column system is used to provide refrigeration to the process. Multiple pressures are chosen for the refrigerant to optimize the cooling curves and thus the efficiency of heat transfer. Some of the helium-depleted liquid is pumped to minimise overall recompression power. All of the returning natural gas streams are recompressed to match the feed pressure if returning to a pipeline, or are recompressed to whatever pressure is required for utilization of the natural gas, e.g. combustion in a gas turbine.

The pressure of the distillation column system is selected to reduce the risk of poor separation resulting from operating at too high of a pressure. To mitigate the increased power demand, a dense fluid expander (DFE) can be used to generate power that can be used in the process by expanding the feed stream to column pressure. Expanding the fluid isentropically through a DFE also produces a lower temperature in the outlet stream than would be produced by expanding isenthalpically though a valve. Using a DFE saves power for an increased capital cost, and must be optimized accordingly. The process can also utilize an expander on one or more of the returning streams to reduce overall net power consumption and provide refrigeration to the process.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the appended figures wherein like numerals denote like elements:

FIG. 1 is a flowsheet depicting a process for the pretreatment, extraction, purification, and liquefaction of helium from a natural gas stream.

FIG. 2 is a flowsheet depicting the helium extraction process according to the present invention.

DETAILED DESCRIPTION

The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. Various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention, as set forth in the appended claims.

The articles “a” or “an” as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.

The term “and/or” placed between a first entity and a second entity includes any of the meanings of (1) only the first entity, (2) only the second entity, and (3) the first entity and the second entity. The term “and/or” placed between the last two entities of a list of 3 or more entities means at least one of the entities in the list including any specific combination of entities in this list. For example, “A, B and/or C” has the same meaning as “A and/or B and/or C” and comprises the following combinations of A, B and C: (1) only A, (2) only B, (3) only C, (4) A and B and not C, (5) A and C and not B, (6) B and C and not A, and (7) A and B and C.

The term “plurality” means “two or more than two.”

The adjective “any” means one, some, or all, indiscriminately of quantity.

The phrase “at least a portion” means “a portion or all.” The “at least a portion of a stream” has the same composition, with the same concentration of each of the species, as the stream from which it is derived.

As used herein, “first,” “second,” “third,” etc. are used to distinguish among a plurality of steps and/or features, and is not indicative of the total number, or relative position in time and/or space, unless expressly stated as such.

All composition values will be specified in mole percent.

The terms “depleted” or “lean” mean having a lesser mole percent concentration of the indicated component than the original stream from which it was formed. “Depleted” and “lean” do not mean that the stream is completely lacking the indicated component.

The terms “rich” or “enriched” mean having a greater mole percent concentration of the indicated component than the original stream from which it was formed.

“Downstream” and “upstream” refer to the intended flow direction of the process fluid transferred. If the intended flow direction of the process fluid is from the first device to the second device, the second device is downstream of the first device. In case of a recycle stream, downstream and upstream refer to the first pass of the process fluid.

The term “dense fluid expander,” abbreviated DFE, also known as a liquid expander, refers to equipment that extracts mechanical work from lowering the pressure of a dense fluid such as a liquid or a supercritical fluid, similar in function to an expander for gases. This expansion is best approximated as an isentropic process, as opposed to a valve which is best approximated as an isenthalpic process.

The term “indirect heat exchange” refers to the process of transferring sensible heat and/or latent heat between two or more fluids without the fluids in question coming into physical contact with one another. The heat may be transferred through the wall of a heat exchanger or with the use of an intermediate heat transfer fluid. The term “hot stream” refers to any stream that exits the heat exchanger at a lower temperature than it entered. Conversely, a “cold stream” is one that exits the heat exchanger at a higher temperature than it entered.

The term “distillation column” includes fractionating columns, rectifying columns, and stripping columns. The distillation column may refer to a single column or a plurality of columns in series or parallel, where the plurality can be any combination of the above column types. Each column may comprise one or more sections of trays and/or packing.

The term “reboiling” refers to partially vaporizing a liquid present in the distillation column, typically by indirect heat exchange against a warmer process stream. This produces a vapor that facilitates mass transfer within the distillation column. The liquid may originate in the bottoms liquid or an intermediate stage in the column. The heat duty for reboiling may be transferred in the distillation column using an in situ reboiler or externally in a heat exchanger dedicated for the purpose or part of a larger heat exchanger system. The vapor-liquid separation also may take place within the distillation column or within an external flash vessel.

The present apparatus and process are described with reference to the figures. In this disclosure, a single reference number may be used to identify a process gas stream and the process gas transfer line that carries said process gas stream. Which feature the reference number refers to will be understood depending on the context.

For the purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

The natural gas feed described in the present invention refers to a gas comprising hydrocarbons, usually originating underground in a geological formation. The natural gas is typically produced at a pressure ranging from about 1 to about 200 bar. All pressures referred to are absolute, not gauge. The pressure of the natural gas is preferably from about 10 to about 100 bar.

The methane content in natural gas typically ranges from about 50% to about 99%. All composition percentages referred to are in volume, or molar, basis, not weight basis.

The nitrogen content in natural gas typically ranges from about 1% to about 50%, or from about 10% to about 35%.

The helium content in natural gas typically ranges from about 0.01% to about 10%. Some embodiments of the present invention are directed to extracting helium from natural gas comprising from about 0.05% to about 1.0%, or from about 0.05% to about 0.2% helium.

FIG. 1 shows the overall process of helium production from a natural gas source. The raw natural gas 1 enters acid gas removal unit A as needed for removal of gases such as CO₂, H₂S and COS that would freeze in the downstream cryogenic units.

The acid gases in stream 2 can be vented to the atmosphere or sent to sulfur removal as needed. There are several options for acid gas removal, including pressure swing adsorption, vacuum swing adsorption, or methanol absorption, which in the following examples presented herein is assumed to be an amine absorber regenerated with steam.

The contaminant-lean natural gas leaving A in stream 3 now contains an acceptably low level of acid gases, typically at a specification of less than about 100 ppmv. If an amine absorber is used, stream 3 will be saturated in water vapor that would solidify in the downstream cryogenic process. Stream 3 would therefore feed dehydration unit D which preferably comprises a temperature swing adsorber (TSA) and a mercury guard bed comprising activated carbon, both well-known in the art for water and mercury removal, respectively. The TSA removes water, CO₂, and aromatics such as benzene, toluene, and xylene (collectively known as BTX). Specifications are set to prevent the formation of a solid phase in the cryogenic process; for example the water specification is often about 1 ppmv.

The impurities are adsorbed and then removed when the TSA is regenerated. Regeneration requires both heat, which can be provided by electrical heaters or process steam, and a process stream to carry the impurities out of the TSA, such as nitrogen or a portion of a helium-depleted stream from the helium extraction unit X. Depending on the pressure required to regenerate the TSA, that stream may be at least a portion of a low-pressure return stream 27 or the helium-depleted natural gas stream 29. Shown in FIG. 1 is the case where a low-pressure stream may be used, in which case stream 27′ is used as the regeneration gas. The impurity-laden natural gas stream 27″ can then be recombined with the remainder of stream 27 prior to recompression. If nitrogen is used to regenerate the TSA, the impurity-laden nitrogen gas may be vented to the atmosphere or sent to further treatment to remove the hydrocarbons as determined by air pollution regulations. When vapor-phase mercury is present in the feed stream, a mercury guard bed is required with the TSA to prevent the vapor-phase mercury from attacking the aluminum in the downstream heat exchangers.

The natural gas feed stream 5 enters the helium extraction unit X, which is the subject of the present invention. The helium extraction unit is a cryogenic process that separates at least 99% of the helium, or at least 99.5% of the helium from the natural gas feed stream, which can contain from 0.05% to 1.0% helium by volume, or from 0.05% to 0.2% helium by volume, to create a crude helium stream, which can contain 4 to 20% helium by volume. The cryogenic process is designed for maximum efficiency at this high helium recovery rate that requires careful heat integration to reduce the overall power requirements of the process.

The low-pressure return stream 27 is recompressed in compressor R if needed to be returned to the pipeline, combusted in a gas turbine, or otherwise utilized as helium-depleted natural gas 29. The low pressure return stream may leave the extraction unit as one or more streams at different pressures. For an example, see streams 27 and 28 in FIG. 2. Higher pressure streams may enter a later stage of compression or bypass compression entirely and be combined with the helium-depleted natural gas.

Crude helium stream 16 is sent to helium purification unit P to produce a pure helium stream 30 with a typical specification of 99.99% or 99.999%. Within the helium purification unit (and therefore not shown in FIG. 1) stream 16 is cooled to cryogenic temperatures, partially condensing the feed stream so that most of the nitrogen and virtually all the methane condenses, leaving a vapor stream with a composition of about 50 to 90% helium, or about 70 to 85% helium, and usually about 80% helium. This vapor stream is warmed and then the hydrogen is removed by a catalytic combustion process before it is fed to a helium PSA. The liquid stream leaving the partial condensation process is reduced in pressure to recover helium by flashing and withdrawing a vapor stream. The vapor stream is warmed, recompressed, and combined with the crude helium stream entering the purifier. Having decreased the pressure of the liquid stream, the resulting lower-temperature liquid stream provides refrigeration to the partial condensation process. This small stream 31 leaves the helium purification process and can be recompressed into the sales gas stream as stream 31′ or vented to atmosphere as stream 31 after passing through a catalytic combustion process to remove the methane if needed. The tail gas from the helium PSA is dried by a TSA to remove the water produced by the catalytic combustion of hydrogen with air or a stream enriched in oxygen, then is recompressed and mixed with stream 16 as it enters the purifier. Also recycled to the feed to the purifier is the helium collected in the “gas bag”, to minimize the overall losses of helium from the process.

Alternate embodiments of the helium purification unit are well known in the art. Blackwell and Kalman (U.S. Pat. No. 3,599,438) describe helium purification in more detail, including the steps of hydrogen removal by catalytic oxidation, dehydration by adsorption, and helium enrichment by partial condensation. Blackwell and Kalman also show the recycle of the intermediate pressure helium stream (16). Kirk-Othmer Encyclopedia of Chemical Technology, “Cryogenic technology,” (2012) also describes alternative helium purification arrangements. For example, FIG. 13 in that chapter shows a process with a single pressure flash in the helium cold end that causes a higher helium loss due to helium dissolved in the liquid stream leaving the system. FIG. 14 in the same chapter shows a different order of operations: partial condensation first, followed by catalytic oxidation and final purification by PSA, where the PSA tail gas is recompressed, dehydrated, and recycled to the partial condensation step. Gottier and Herron (U.S. Pat. No. 5,017,204) describe a helium purification cycle employing a dephlegmator that combines heat transfer and mass transfer steps into a single heat exchanger. Any of these methods, or similar purification methods, may be employed to generate a pure helium product from a crude helium stream.

The pure helium stream 30 can be sold as a gaseous product, but more commonly it is liquefied in helium liquefier L to produce a liquid helium stream 32 that can be transported long distances more efficiently. The liquefier also removes traces of neon if present. The liquefier may use liquid nitrogen for refrigeration at the warm end of the process, provided by a small nitrogen generator or imported by truck as liquid, or it may use any other refrigeration option known in the art. The cold end of the process typically uses recycled helium in a heat pump arrangement for refrigeration.

If desired, at least a portion of the acid gas stream 2′ and/or the tail gas 31′ can be mixed with the helium-depleted natural gas 29 prior to recompression or at an interstage in the recompression. This can be advantageous if the helium-depleted natural gas was designed for a given mass flow rate, such as for a gas turbine. If there is a small amount of H₂S present in the acid gas stream, then recompression and dilution may avoid the complications of venting H₂S-containing CO₂, or the expense of oxidizing the H₂S, or the cost of a tall vent stack. Similarly, recompression of stream 31′ can avoid the added cost of oxidizing the remaining methane in the tail gas stream if needed prior to venting.

FIG. 2 shows the helium extraction unit X in detail. The natural gas feed stream 5 is fed to a heat exchanger 101, after leaving the pretreatment units A and D in FIG. 1. The heat exchanger is typically a brazed aluminium plate-fin heat exchanger, common to the cryogenic industry, and can be configured as one or more heat exchangers in series or parallel. The stream is cooled in the heat exchanger against streams returning from the cryogenic distillation section, at least partially condensed, and exits the heat exchanger as cooled natural gas feed 6. If needed, the pressure of stream 6 may be reduced in order to achieve a good separation in the distillation column. The parameter for achieving good separation may be the ratio of liquid phase density to vapor phase density, where the desired ratio is greater than 4, or greater than 6, or greater than 8. The parameter may also be the liquid phase surface tension, where the desired value is greater than 0.5 dyne/cm, or greater than 1 dyne/cm, or greater than 2 dyne/cm. If pressure reduction is required, it is shown in FIG. 2 as occurring in valve 102, but can also be achieved by a dense fluid expander. The column feed stream 7 then enters distillation column 103, preferably at the top stage.

The distillation column 103 separates the helium from the column feed stream 7, which leaves the top of the column as helium-enriched overhead vapor 8. The distillation column requires a reboiler, which is shown in FIG. 2 as an external reboiler. In this configuration liquid stream 9 leaves the bottom of the column and then is heated indirectly by the natural gas feed 5 in the heat exchanger 101. The partially vaporized stream 10 is then separated in a reboiler separator 104. The distillation column 103, the reboiler separator 104, and the portion of heat exchanger 101 used for transferring heat to stream 9 compose the distillation column system. Vapor stream 11 is returned to the distillation column 103 and helium-depleted bottoms liquid exits the distillation column system as stream 12.

The distillation column system is shown in FIG. 2 with an external reboiler arrangement, where 104 is the reboiler separator. The reboiler can also be internal to the column, or the external reboiler can be a separate heat exchanger rather than integrated into a multiple-stream heat exchanger with other hot and cold streams as shown as 101 in FIG. 2. The reboiler provides vapor feed to the bottom of the column by boiling part of the liquid leaving the bottom of the column as stream 9. As known in the art, this can be done in several ways. A reboiler, such as a thermosyphon reboiler, could sit in the liquid sump to boil liquid within the sump. In that case a stream with a temperature between that of streams 5 and 6 would be fed to the reboiler to provide the required heat and the liquid stream leaving the column sump would have the same conditions as stream 12 in FIG. 2. The distillation column system can employ one of the reboiler configurations described above or any other known reboiler.

The helium-enriched overhead vapor 8 is then partially condensed in heat exchanger 105. The partially condensed overhead 13 enters overhead separator 106. This overhead separator 106 may be a simple flash vessel or a distillation column with multiple stages. The overhead from 106 is the crude helium vapor stream 14. The crude helium vapor stream 14 provides refrigeration by traveling through both heat exchangers 105 and 101 before leaving the helium extraction unit as stream 16. This crude helium vapor 14 is now at a high enough concentration, typically 4% to 20% by volume, to enter a helium purifier, shown as unit P in FIG. 1.

The recycle liquid stream 17 exits the bottom of overhead separator 106 and is returned to the distillation column 103. Although stream 17 appears in the same location in the flow sheet as would a reflux stream for a conventional distillation process, the recycle liquid in the present invention is unsuitable for providing reflux for two reasons. First, the flow of stream 17 is small compared to the flow in the column feed 7, unlike a reflux stream that must have a liquid flow rate high enough to wash the vapor flowing up the column. Because the recycle liquid stream 17 has a relatively small flow rate, it does not affect the separation and is only returned to the distillation column to recover the helium contained in stream 17. Second, the distillation column 103 operates as a stripping column with the column feed entering at the top stage. The recycle liquid 17 can enter at the top stage or any lower stage, so it does not have the opportunity to wash the vapor leaving the top stage.

The pressure in the overhead separator 106 must be kept as close as possible to the pressure of the distillation column system such that the liquid head pressure in stream 17 is sufficient to overcome the pressure drop and flow into the distillation column 103. This lowers the overall power consumption of the process because the crude helium vapor stream from the overhead separator 106 thus requires no recompression. Note that the higher pressure in the overhead separator results in more helium being trapped in the stream 17, but this liquid-phase helium is recovered by recycling stream 17 back to the distillation column 103.

The helium-depleted bottoms liquid 12 may be split into at least two streams, each of which provides cooling at a different pressure and so different temperature. Stream 12 may be split into up to as many streams as one more than the number of stages of compression available in the recompressor R. This is because each stage of compression can accept one stream at its suction pressure, and one additional stream may bypass R if it is at the same pressure as the outlet of R. In the embodiment shown in FIG. 2, stream 12 is split into three streams: 18, 21, and 23. Using the product streams to refrigerate the process is known as auto-refrigeration, and improves efficiency compared to external refrigeration. Using multiple pressure levels of the returning process streams minimizes the temperature differences throughout the heat exchanger system, improving efficiency and resulting in a lower overall recompression power requirements. The first helium-depleted bottoms fraction 18 is reduced in pressure to produce stream 19. The pressure reduction is shown as valve 105 but could also be achieved with a DFE. Stream 19 provides the refrigeration to partially condense stream 8 in heat exchanger 105, after which stream 20 provides more refrigeration to heat exchanger 101. The second helium-depleted bottoms fraction 21 may be reduced in pressure to produce stream 22 if needed for additional refrigeration. If required, the pressure reduction could be effected in valve 107 or with a DFE. Stream 22 provides refrigeration to heat exchanger 101. Because it is let down to an intermediate pressure greater than the pressure of stream 19, the temperature of stream 22 is not as cold as stream 19. The third helium-depleted bottoms fraction 23 can be increased in pressure in pump 108 to produce stream 24. Stream 24 then provides refrigeration by being vaporized in heat exchanger 101. Pumping a liquid stream before vaporizing it, as shown herein, is more efficient than vaporizing a liquid stream and then compressing the vapor because liquids are effectively incompressible.

After stream 22 is warmed in heat exchanger 101, the resulting warmed second helium-depleted bottoms fraction 25 may be expanded in expander 109, if desired, which both cools the stream and generates power. The resulting expanded second helium-depleted bottoms fraction 26 can be returned to heat exchanger 101 to provide more cooling, then be combined with stream 20, and finally exit the heat exchanger as low-pressure return stream 27. Stream 27 is then recompressed in return compressor R. Stream 24 exits the heat exchanger 101 as medium-pressure return stream 28, which can be recompressed by feeding an interstage of return compressor R. Depending on the pressure required in the final helium-depleted natural gas product, pump 108 could increase the pressure of stream 23 to a high enough level that no further compression is needed.

Heat exchangers 101 and 105 represent a heat exchanger system, which in various embodiments of the invention may be a single heat exchanger or be split into two or more heat exchangers in series or parallel. For instance, the heat exchanger 101 may be divided into two separate heat exchangers at the point the expanded second helium-depleted bottoms fraction 26 is returned to the exchanger and mixed with stream 20 as it returns from 105. It may also be that the duty required for the reboiler is provided by a separate heat exchanger either in parallel with 101 or at the cold end of 101, exchanging heat solely between stream 6 and stream 9 to simplify the operation of the distillation column system. In general, the more integrated the heat exchanger system is, the more efficient the heat exchange is between all of the desired streams. However, the heat exchanger is often divided, which sacrifices efficiency, because a small increase in overall power consumption allows an advantage such as simplified operation, a smaller heat exchanger system, a simpler design of the heat exchanger system, or the reduction of risk to the process.

Return compressor R can be a single compressor with one or more stages, with or without intercoolers between stages, or a plurality of compressors in series or parallel. In the series arrangement, stream 27 could enter the first of the compressors and stream 28 could enter a compressor further along the series. In a parallel arrangement, separate compressors could compress streams 27 and 28 to the desired final discharge pressure. The recompressed gas exits the helium extraction unit as helium-depleted natural gas stream 29, which can then be fed to a pipeline, combusted, or otherwise utilized. If waste streams 2′ and/or 31′ are to be recompressed and combined with the helium-depleted natural gas stream, they are also fed to R.

There are situations where recompression of the medium-pressure return stream may not be required. The pressures of the return streams 20, 22, and 24 must all be less than the pressure of feed stream 5 because the return streams must boil at a pressure lower than the feed stream condenses at to allow efficient operation of heat exchanger 101. If the desired pressure of stream 29 is less than the pressure of stream 5, then stream 24 may be pumped to a pressure equal to that of stream 29 and not need further compression. In that case, the medium-pressure return stream 28′ may instead bypass the return compressor and be mixed directly with stream 29.

Certain embodiments and features of the invention have been described using a set of numerical upper limits and a set of numerical lower limits. For the sake of brevity, only certain ranges are explicitly disclosed herein. However, it should be appreciated that ranges from any lower limit to any upper limit are contemplated unless otherwise indicated. Similarly, ranges from any lower limit may be combined with any other lower limit to recite a range not explicitly recited, and ranges from any upper limit may be combined with any other upper limit to recite a range not explicitly recited. Further, a range includes every point or individual value between its end points even though not explicitly recited. Thus, every point or individual value may serve as its own lower or upper limit combined with any other point or individual value or any other lower or upper limit, to recite a range not explicitly recited. All numerical values are “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

Aspects of the present invention include:

-   #1: A process for recovering helium from a natural gas feed     comprising methane, nitrogen, and helium, said process comprising: -   cooling said natural gas feed to produce a cooled natural gas feed     which is at least partially condensed; -   separating the cooled natural gas feed in a distillation column     system to produce a helium-enriched overhead vapor and a     helium-depleted bottoms liquid; -   cooling said helium-enriched overhead vapor to produce a partially     condensed overhead stream; -   separating said partially condensed overhead stream in an overhead     separator to produce a crude helium vapor and a recycle liquid; -   expanding at least a portion of the helium-depleted bottoms liquid     to produce a first helium-depleted bottoms fraction; -   wherein cooling duty for cooling said helium-enriched overhead vapor     is provided at least in part by indirect heat exchange with said     first helium-depleted bottoms fraction. -   #2. A process according to #1 wherein the pressure of said cooled     natural gas feed is reduced to achieve a ratio of liquid to vapor     density in the distillation column greater than 4. -   #3. A process according to any of #1 to #2 wherein the pressure of     said cooled natural gas feed is reduced to achieve a liquid phase     surface tension in the distillation column greater than 0.5 dyne/cm. -   #4. A process according to any of #1 to #3 wherein the difference     between the pressure of the top of the distillation column system     and the pressure of said overhead separator is no more than 1 bar. -   #5. A process according to any of #1 to #4 wherein the re-boiling     duty for said distillation column system is provided at least in     part by indirect heat exchange with the natural gas feed. -   #6. A process according to any of #1 to #5 wherein said recycle     liquid is introduced to the distillation column. -   #7. A process according to #6 wherein the recycle liquid is     introduced to the distillation column at the same or lower stage as     the location where the cooled natural gas is fed to the distillation     column. -   #8. A process according to any of #1 to #7 further comprising the     step of expanding at least a portion of said helium-depleted bottoms     liquid to produce a second helium-depleted bottoms fraction. -   #9. A process according to #8 wherein the pressure of said second     helium-depleted bottoms fraction is higher than the pressure of said     first helium-depleted bottoms fraction. -   #10. A process according to any of #8 to #9 further comprising the     steps of warming said second helium-depleted bottoms fraction to     provide at least a portion of the refrigeration to cool and condense     said natural gas feed and produce a warmed second helium-depleted     bottoms fraction; -   and expanding said warmed second helium-depleted bottoms fraction to     provide power and produce an expanded second helium-depleted bottoms     fraction. -   #11. A process according to any of #8 to #10 further comprising     combining and compressing said first and second helium-depleted     bottoms fractions, or streams derived therefrom, to produce a     helium-depleted natural gas stream. -   #12. A process according to any of #8 to #11 further comprising the     steps of pressurizing at least a portion of said helium-depleted     bottoms liquid to produce a third helium-depleted bottoms fraction; -   and warming said third helium-depleted bottoms fraction to provide     at least a portion of the refrigeration to cool and condense said     natural gas feed. -   #13. A process according to #12 further comprising combining and     compressing said first, second, and third helium-depleted bottoms     fraction, or streams derived therefrom, to produce a helium-depleted     natural gas stream. -   #14. A natural gas processing plant for recovering helium from a     natural gas feed comprising methane, nitrogen, and helium, said     plant comprising: -   a heat exchanger system; -   a distillation column system comprising a vapor outlet and a liquid     outlet; -   a first conduit for transferring a cooled natural gas feed from said     heat exchanger system to said distillation column; -   a second conduit for transferring a helium-enriched overhead vapor     from said vapor outlet of said distillation column to said heat     exchanger system; -   an overhead separator comprising a vapor outlet and a liquid outlet; -   a third conduit for transferring a partially condensed overhead from     said heat exchanger system to said overhead separator; -   a fourth conduit for transferring a first helium-depleted bottoms     fraction from said liquid outlet of said distillation system to said     heat exchanger system; -   Wherein said fourth conduit comprises a pressure reduction device. -   #15. A natural gas processing plant according to #14 wherein said     first conduit comprises a pressure reduction device. -   #16. A natural gas processing plant according to any of #14 to #15     further comprising a fifth conduit for transferring a recycle liquid     from said liquid outlet of said overhead separator to said     distillation column. -   #17. A natural gas processing plant according to #16 wherein said     fifth conduit connects to said distillation column at the same stage     as or a lower stage than where said first conduit connects to said     distillation column. -   #18. A natural gas processing plant according to any of #14 to #17     further comprising a sixth conduit for transferring a second     helium-depleted bottoms fraction from said liquid outlet of said     distillation system to said heat exchanger system, wherein said     sixth conduit further comprises a pressure reduction device. -   #19. A natural gas processing plant according to #18 further     comprising: an expander; -   a seventh conduit for transferring a warmed second helium-depleted     bottoms fraction from said heat exchanger to said expander; -   and an eighth conduit for transferring an expanded second     helium-depleted bottoms fraction from said expander to said heat     exchanger system. -   #20. A natural gas processing plant according to any of #18 to #19     further comprising: -   a pump; -   a ninth conduit for transferring a third helium-depleted bottoms     fraction from said liquid outlet of said distillation system to said     pump; -   and a tenth conduit for transferring a pressurized third     helium-depleted bottoms fraction from said pump to said heat     exchanger system. -   #21. A natural gas processing plant according to any of #18 to #20     further comprising: -   a return compressor; -   and an eleventh conduit for transferring a low-pressure return     stream from said heat exchanger system to said return compressor. -   #22. A natural gas processing plant according to #21 further     comprising a twelfth conduit for transferring a medium-pressure     return stream from said heat exchanger system to said return     compressor. -   #23. A natural gas processing plant according to #21 further     comprising: -   a mixing device; -   a thirteenth conduit for transferring a compressed helium-depleted     natural gas stream from said return compressor to said mixing     device; -   and a fourteenth conduit for transferring a medium-pressure return     stream from said heat exchanger system to said mixing device.

EXAMPLE 1

A computer simulation of the process of FIGS. 1 and 2 was carried out in Aspen Plus, a commercially available process simulation software package. The feed stream of natural gas contains 35% nitrogen and 0.14% helium. Key stream parameters such as composition, pressure, temperature, and flow rate, are shown in Table 1, along with total power consumption.

For purposes of Example 1, two changes were made to the process depicted in FIGS. 1 and 2. This example assumes that steam 31′ of FIG. 1 is recompressed with the helium-depleted natural gas stream, but stream 2 is vented to atmosphere. This example also assumes that stream 5 of FIG. 2 is cooled, condensed, and expanded across a DFE in place of valve 102.

As shown in Table 1, the helium extraction unit X produces a crude helium stream 16 with greater than 12% helium, rich enough to feed the helium purification unit P, while maintaining 99.9% recovery in the helium extraction unit. Recovery in unit X is defined as the helium contained in stream 16 leaving the unit divided by the helium contained in stream 5 entering the unit. This high recovery is possible because recycle liquid stream 17, which holds 6.8% of the helium contained in the helium-enriched condensed overhead stream 12, is returned to the distillation column. In known processes that further concentrate the distillation column overhead, that liquid-phase helium would be lost because the equivalent of stream 17 would be routed to the equivalent of helium-depleted natural gas stream 29. The 99.9% helium recovery in the helium extraction unit X allows an overall helium recovery of 99.6% due the small loss of helium in stream 31′, where the overall helium recovery is defined as the helium contained in pure helium stream 30 divided by the helium contained in raw natural gas stream 1.

This process provides flexibility over the crude helium stream 16 composition. The helium mole fraction of stream 16 can be increased by either increasing the flow rate or decreasing the pressure of the low pressure return stream 19. Either option results in a higher concentration of helium in stream 16 at the cost of an increased power requirement to compress stream 27.

If the waste stream from the helium purification process were to be vented as stream 31, an optimization that minimizes power would increase the flow rate of methane in stream 16 to avoid recompression in compressor R. The optimization would need to include the the value of methane in the vent 31 to balance the increase in stream 16.

TABLE 1 Stream Component Composition 1 2 5 8 12 14 17 19 He mol % 0.14 0.00 0.15 2.81 0.00 12.80 0.24 0.00 N₂ mol % 35.09 0.00 36.74 75.26 36.22 81.45 73.67 36.22 CO₂ mol % 4.01 88.79 0.00 0.00 0.00 0.00 0.00 0.00 CH₄ mol % 60.16 0.00 62.99 21.78 63.65 5.17 26.05 63.65 C₂H₆ mol % 0.10 0.00 0.10 0.00 0.11 0.00 0.00 0.11 C₃H₈ mol % 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.01 H₂O mol % 0.47 11.14 0.00 0.00 0.00 0.00 0.00 0.00 H₂ mol % 0.01 0.00 0.01 0.15 0.00 0.58 0.04 0.00 Temperature ° C. 67.2 58.9 26.7 −140.6 −139.3 −155.9 −155.9 −160.1 Pressure bar (abs) 39.3 1.7 37.6 19.7 19.8 19.7 19.7 5.5 Flowrate (total) kmol/hr 22679.6 1024.4 21662.9 1212.6 21415.0 247.8 964.7 1054.3 Stream Component Composition 22 24 25 26 31′ 29 16 30 He mol % 0.00 0.00 0.00 0.00 0.01 0.00 12.80 100.00 N₂ mol % 36.22 36.22 36.22 36.22 94.06 36.81 81.45 0.00 CO₂ mol % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CH₄ mol % 63.65 63.65 63.65 63.65 5.90 63.07 5.17 0.00 C₂H₆ mol % 0.11 0.11 0.11 0.11 0.00 0.11 0.00 0.00 C₃H₈ mol % 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 H₂O mol % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 H₂ mol % 0.00 0.00 0.00 0.00 0.02 0.00 0.58 0.00 Temperature ° C. −139.3 −137.3 −36.1 −92.3 26.4 67.2 25.0 51.1 Pressure bar (abs) 19.8 34.9 19.3 5.0 1.8 38.3 19.2 17.4 Flowrate (total) kmol/hr 1216.5 19144.3 1216.5 1216.5 217.4 21632.4 247.8 31.7 Recompression Power 8.95 MW Pump Power 0.71 MW Expander Power −0.47 MW DFE Power −0.20 MW Total Net Power 9.01 MW

EXAMPLE 2

A computer simulation of the process of FIGS. 1 and 2 was carried out in Aspen Plus, a commercially available process simulation software package. The feed stream of natural gas contains 10% nitrogen and 0.065% helium. Table 2 gives the conditions for the streams in FIGS. 1 and 2 along with total power consumption.

For purposes of Example 2, two changes were made to the process depicted in FIGS. 1 and 2. This example assumes that steam 31′ of FIG. 1 is recompressed with the natural gas return, but stream 2 is vented to atmosphere. This examples also assumes that stream 5 of FIG. 2 is cooled, condensed, and expanded across a DFE in place of valve 102.

Example 2 shares many of the same features as Example 1, such as high overall helium recovery, but differs in the nitrogen content of the feed. The lower nitrogen content in Example 2 results in higher temperatures in the distillation column 103, as shown by a stream 8 that is about 20° C. warmer than its counterpart in Example 1. Because the distillation column does not require as cold of a temperature, stream 19 does not need to be let down to as low of a pressure: 7.7 bar as opposed to 5.5 bar. Stream 19 operating at a higher pressure reduces the recompression duty, resulting in a lower net power of 7.75 MW compared to 9.01 MW in Example 1.

TABLE 2 Stream Component Composition 1 2 5 8 12 14 17 19 He mol % 0.065 0.000 0.068 1.404 0.000 4.721 0.055 0.000 N₂ mol % 10.03 0.00 10.51 36.45 9.72 64.04 25.24 9.72 CO₂ mol % 4.01 88.79 0.00 0.00 0.00 0.00 0.00 0.00 CH₄ mol % 85.29 0.00 89.30 61.97 90.16 30.71 74.67 90.16 C₂H₆ mol % 0.10 0.00 0.11 0.00 0.11 0.00 0.00 0.11 C₃H₈ mol % 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.01 H₂O mol % 0.47 11.14 0.00 0.00 0.00 0.00 0.00 0.00 H₂ mol % 0.01 0.00 0.01 0.17 0.00 0.53 0.02 0.00 Temperature ° C. 67.2 58.9 26.7 −120.7 −119.2 −135.2 −135.2 −136.9 Pressure bar (abs) 39.3 1.7 37.6 19.7 19.8 19.7 19.7 7.7 Flowrate (total) kmol/hr 22679.6 1025.2 21662.1 1079.5 21350.2 311.9 767.6 741.4 Stream Component Composition 22 24 25 26 31′ 29 16 30 He mol % 0.000 0.000 0.000 0.000 0.002 0.000 4.721 99.995 N₂ mol % 9.72 9.72 9.72 9.72 67.89 10.53 64.04 0.00 CO₂ mol % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CH₄ mol % 90.16 90.16 90.16 90.16 32.07 89.35 30.71 0.00 C₂H₆ mol % 0.11 0.11 0.11 0.11 0.00 0.11 0.00 0.00 C₃H₈ mol % 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 H₂O mol % 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 H₂ mol % 0.00 0.00 0.00 0.00 0.02 0.00 0.53 0.00 Temperature ° C. −119.2 −117.1 −32.0 −78.2 23.3 67.2 25.0 51.1 Pressure bar (abs) 19.8 35.0 19.3 7.2 1.8 38.3 19.2 17.4 Flowrate (total) kmol/hr 1319.6 19289.1 1319.6 1319.6 298.6 21648.8 311.9 14.7 Recompression Power 7.65 MW Pump Power 0.77 MW Expander Power −0.43 MW DFE Power −0.24 MW Total Net Power 7.75 MW

While the principles of the invention have been described above in connection with preferred embodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of the invention. 

We claim:
 1. A process for recovering helium from a natural gas feed comprising methane, nitrogen, and helium, said process comprising: cooling said natural gas feed to produce a cooled natural gas feed which is at least partially condensed; separating the cooled natural gas feed in a distillation column system to produce a helium-enriched overhead vapor and a helium-depleted bottoms liquid; cooling said helium-enriched overhead vapor to produce a partially condensed overhead stream; separating said partially condensed overhead stream in an overhead separator to produce a crude helium vapor and a recycle liquid; expanding at least a portion of the helium-depleted bottoms liquid to produce a first helium-depleted bottoms fraction; wherein cooling duty for cooling said helium-enriched overhead vapor is provided at least in part by indirect heat exchange with said first helium-depleted bottoms fraction.
 2. Process of claim 1 wherein the pressure of said cooled natural gas feed is reduced to achieve a ratio of liquid to vapor density in the distillation column greater than
 4. 3. Process of claim 1 wherein the pressure of said cooled natural gas feed is reduced to achieve a liquid phase surface tension in the distillation column greater than 0.5 dyne/cm.
 4. Process of claim 1 wherein the difference between the pressure of the top of the distillation column system and the pressure of said overhead separator is no more than 1 bar.
 5. Process of claim 1 wherein the re-boiling duty for said distillation column system is provided at least in part by indirect heat exchange with the natural gas feed.
 6. Process of claim 1 wherein said recycle liquid is introduced to the distillation column.
 7. Process of claim 6 wherein the recycle liquid is introduced to the distillation column at the same or lower stage as the location where the cooled natural gas is fed to the distillation column.
 8. Process of claim 1 further comprising the step of expanding at least a portion of said helium-depleted bottoms liquid to produce a second helium-depleted bottoms fraction.
 9. Process of claim 8 wherein the pressure of said second helium-depleted bottoms fraction is higher than the pressure of said first helium-depleted bottoms fraction.
 10. Process of claim 8 further comprising the steps of warming said second helium-depleted bottoms fraction to provide at least a portion of the refrigeration to cool and condense said natural gas feed and produce a warmed second helium-depleted bottoms fraction; and expanding said warmed second helium-depleted bottoms fraction to provide power and produce an expanded second helium-depleted bottoms fraction.
 11. Process of claim 8 further comprising combining and compressing said first and second helium-depleted bottoms fractions, or streams derived therefrom, to produce a helium-depleted natural gas stream.
 12. Process of claim 8 further comprising the steps of pressurizing at least a portion of said helium-depleted bottoms liquid to produce a third helium-depleted bottoms fraction; and warming said third helium-depleted bottoms fraction to provide at least a portion of the refrigeration to cool and condense said natural gas feed.
 13. Process of claim 12 further comprising combining and compressing said first, second, and third helium-depleted bottoms fraction, or streams derived therefrom, to produce a helium-depleted natural gas stream.
 14. A natural gas processing plant for recovering helium from a natural gas feed comprising methane, nitrogen, and helium, said plant comprising: a heat exchanger system; a distillation column system comprising a vapor outlet and a liquid outlet; a first conduit for transferring a cooled natural gas feed from said heat exchanger system to said distillation column; a second conduit for transferring a helium-enriched overhead vapor from said vapor outlet of said distillation column to said heat exchanger system; an overhead separator comprising a vapor outlet and a liquid outlet; a third conduit for transferring a partially condensed overhead from said heat exchanger system to said overhead separator; a fourth conduit for transferring a first helium-depleted bottoms fraction from said liquid outlet of said distillation system to said heat exchanger system; Wherein said fourth conduit comprises a pressure reduction device.
 15. The natural gas processing plant of claim 14 wherein said first conduit comprises a pressure reduction device.
 16. The natural gas processing plant of claim 14 further comprising a fifth conduit for transferring a recycle liquid from said liquid outlet of said overhead separator to said distillation column.
 17. The natural gas processing plant of claim 16 wherein said fifth conduit connects to said distillation column at the same stage as or a lower stage than where said first conduit connects to said distillation column.
 18. The natural gas processing plant of claim 14 further comprising a sixth conduit for transferring a second helium-depleted bottoms fraction from said liquid outlet of said distillation system to said heat exchanger system, wherein said sixth conduit further comprises a pressure reduction device.
 19. The natural gas processing plant of claim 18 further comprising: an expander; a seventh conduit for transferring a warmed second helium-depleted bottoms fraction from said heat exchanger to said expander; and an eighth conduit for transferring an expanded second helium-depleted bottoms fraction from said expander to said heat exchanger system.
 20. The natural gas processing plant of claim 18 further comprising: a pump; a ninth conduit for transferring a third helium-depleted bottoms fraction from said liquid outlet of said distillation system to said pump; and a tenth conduit for transferring a pressurized third helium-depleted bottoms fraction from said pump to said heat exchanger system.
 21. The natural gas processing plant of claim 18 further comprising: a return compressor; and an eleventh conduit for transferring a low-pressure return stream from said heat exchanger system to said return compressor.
 22. The natural gas processing plant of claim 21 further comprising a twelfth conduit for transferring a medium-pressure return stream from said heat exchanger system to said return compressor.
 23. The natural gas processing plant of claim 21 further comprising: a mixing device; a thirteenth conduit for transferring a compressed helium-depleted natural gas stream from said return compressor to said mixing device; and a fourteenth conduit for transferring a medium-pressure return stream from said heat exchanger system to said mixing device. 